Exemplary embodiments of the invention relate to a waveguide radiator having a slotted waveguide with a plurality of slots provided in the waveguide. Exemplary embodiments of the invention further relate to an array antenna radiator and a synthetic aperture radar system.
Waveguide radiators or array antenna radiators (in the literature also referred to as radiators or subarrays) are used, for example, in phased-array antennas of synthetic aperture radar (SAR) systems with single or dual polarization. Up to now, so-called microstrip patch antennas or slotted waveguide antennas are used as radiators.
Microstrip patch antennas exhibit high electrical losses and, due to their electrical feed network, cannot be efficiently implemented in greater radiator lengths than approximately seven wavelengths (in the X-band approximately 20 cm). In the case of an active antenna with distributed generation of the HF transmitting power by so-called T/R modules (transmit/receive modules) there is also the problem of dissipating the heat of the active modules, which are located on the rear side of the radiators, to the front.
The slotted waveguide antennas, on the other hand, are limited by their electrically resonant behavior in the achievable relative bandwidth (<5%). Moreover, they require high manufacturing accuracy and can be produced as dual-polarized array antennas only with very high costs. Concepts used in the prior art are waveguides with inner webs and longitudinal slots for vertical polarization, and rectangular waveguides with diagonally inserted wires and transversal slots for horizontal polarization. The problem here is the required transitions of the connected coaxial cables into the waveguides.
German patent document DE 10 2006 057 144 A1 discloses a waveguide radiator comprising a slotted waveguide in which an additional inner conductor, a so-called barline, is provided. This inner conductor is specially shaped in a polarization-dependent manner in order to excite all slots of the waveguide with identical phase. In contrast to conventional slotted waveguides, the propagation modes are no longer dispersive but correspond to those in coaxial lines, i.e., TEM modes. Hereby, the bandwidth can increase. Moreover, the cross-sections of the waveguides can be considerably reduced in size since no lower limiting frequency (so-called cutoff frequency) exists in the case of TEM modes. Coupling can take place by a direct coaxial transition, which can be implemented in a mechanically simple manner, for example by commercially available SMA installation sockets.
Exemplary embodiments of the invention are directed to a waveguide radiator that is functionally and/or structurally improved. The waveguide radiator is broadband and is producible in an efficient and cost-effective manner so that that it can be used for building a planar array antenna that can be used in space-based or aircraft-based synthetic aperture radar (SAR) systems.
In accordance with exemplary embodiments of the invention a waveguide radiator comprises a slotted waveguide radiator (waveguide) having a plurality of transversal or longitudinal slots provided in the waveguide. If the waveguide has transversal slots, the direction of the radiated polarization of the waveguide corresponds to the longitudinal direction of the waveguide. If the slotted waveguide has longitudinal slots, the direction of the radiated polarization of the waveguide corresponds to the transverse direction of the waveguide. Depending on the alignment of the slots, thus, either horizontally or vertically polarized waves can be radiated. The additional inner conductor fitted in the waveguide is shaped independently of the alignment of the slots in such a manner that the result is a feed according to the traveling wave principle, wherein all slots of the waveguide can be excited with identical phase.
Due to the inner conductor (so-called barline) located in the interior of the waveguide, a dispersion-free, transversal electromagnetic propagation mode (TEM mode) is supported. The inner conductor is shaped in a polarization-dependent manner to be specifically able to excite either longitudinal or transversal slots. Compared to the waveguide radiator described in German patent document DE 10 2006 057 144 A1, the waveguide radiator of the present invention has a significantly greater bandwidth.
In order to secure the inner conductor, a layer of dielectric material is placed in the waveguide, on the surface of which the inner conductor is fitted, for example by adhesive bonding.
The height or thickness of the dielectric layer along the waveguide is not uniform but has an individually shaped height profile. By means of the height profile and the shape of the inner conductor, the amplitude and phase of the electric field strength in the slots along the waveguide can be specifically influenced so that any desired aperture illuminations can be implemented, for example, in order suppress side lobes in the antenna radiation pattern below a predetermined value. In the same manner, a homogenous amplitude and phase occupancy along the waveguide can be achieved, for example, in order to maximize the antenna gain and to minimize the full width half maximum.
Each slot of the waveguide radiator can have individual geometric dimensions. However, it is to be understood that the waveguide radiator can have either only longitudinal or only transversal slots.
The specific shape of the inner conductor is composed of repetitive sections of similar geometry along the waveguide. The length of these sections is identical here to the spacing of adjacent slots along the waveguide. The additional inner conductor can be formed in particular from alternately arranged straight and twisted conductor sections.
One firm with respect to the resonant feed with a standing wave is an additional quarter-wave transformer that is located in each of the repetitive sections. This quarter-wave transformer is implemented by tapering the inner conductor, i.e., reducing the conductor width. The length of this taper or the conductor width reduction is preferably selected such that it corresponds to an electrical path length of exactly the quarter of a line wavelength. The reduction of the conductor width effects an increase of the wave impedance along the tapered section. By the quarter-wave transformers implemented in this manner, reflection points are compensated which otherwise would occur at these positions.
In the region of the ends of the waveguide, the inner conductor can have a straight section as an open stub.
While the radiator described in German patent document DE 10 2006 057 144 A1 uses a feed with standing wave, the waveguide according to the invention uses a so-called traveling wave feed.
Coupling a signal can take place in the center of the waveguide radiator by a galvanically coupled coaxial transition, wherein the inner conductor of a connected coaxial cable (e.g., via SMA, SMP connection) is directly connected to the feed point of the inner conductor. The outer conductor of the connected coaxial cable is directly connected to the wall of the waveguide.
The feed point can be slightly shifted in the transverse direction so as to thereby enable the transition at a suitable place to a circuit board attached on the rear side of the radiator.
In the case of slotted waveguide having transverse slots, the feed point of the waveguide can be shifted with respect to the geometric center of the waveguide in the longitudinal direction. In a specific implementation, the shift can be approximately 6 to 7 mm, wherein said shift depends on the wavelength or frequency of the signal to be generated.
In another configuration of a slotted waveguide having transverse slots, the feed point of the waveguide can be arranged in the waveguide in such a manner that the electric phase at the positions of slots is identical at center frequency.
In the case of a slotted waveguide having longitudinal slots, the additional inner conductor has a feed point which, in the longitudinal direction of the slotted waveguide, is arranged in the geometric center. It can also be provided that the slotted waveguide with the additional inner conductor is formed mirror-symmetrically around the feed point.
Overall, it is achieved that the wave fed at the feed point of the radiator can propagate in the center of the radiator without reflection up to the ends of the inner conductor.
The invention has the advantage that in contrast to the resonant feed, significantly greater band widths can be implemented. The advantages mentioned in German patent document DE 10 2006 057 144 A1 regarding conventional slotted waveguides remain valid such as, e.g., no dispersion, size reduction of the cross-section, no cutoff frequency, robustness with respect to manufacturing tolerances, possibility of greater radiator lengths, low production costs, short production time, problem-free transition to coaxial cable, high power can be fed, low ohmic losses, high cross-polar suppression.
Developing the waveguide radiators, in particular determining the exact geometric dimensions of the inner conductor and the slots is performed by means of electromagnetic simulation methods. The behavior of the radiator described here can also approximately be described by network models with suitable equivalent circuit diagrams. These models are normally used in a first step in order to optimize the dimensions of the elements present in the equivalent circuit diagram. In the second step, these dimensions are then translated into suitable geometric parameters. For this, commercially available software packets can be used that calculate the electromagnetic behavior of the actual geometry (3D model) by means of a flu wave analysis.
An array antenna radiator according to the invention comprises one or a plurality of slotted waveguides having transverse slots and one or a plurality of slotted waveguides having longitudinal slots of the kind described above. In one configuration, the slotted waveguides can be arranged side-by-side in the transverse direction, wherein a waveguide having transverse slots and a waveguide having longitudinal slots alternately adjoin each other. Here, the waveguides, i.e., all waveguides, preferably have an identical length.
The waveguides having transverse slots can be offset upwards with respect to the waveguides having longitudinal slots so that a step-like structure of the array antenna radiator is created. The top side here is that side of a respective waveguide on which the slots are located on the waveguides.
A synthetic aperture radar system, in particular a high-resolution synthetic aperture radar system comprises at least one array antenna radiator of the above-described kind.
The absolute values and dimensions indicated below are merely exemplary values and do not limit the invention in any way to such dimensions. The illustrations show the invention only schematically and are in particular not to be considered as being true to scale.